From Wikipedia, the free encyclopedia

A simplified representation of a helium atom, the second smallest of all atoms,
having an estimated (calculated) radius of 31 picometres[1]

For examples of things measuring between one
and ten picometres, see 1 picometre.

A picometre (symbol pm) is a
unit of length in the metric system, equal to one trillionth,
i.e. (1/1,000,000,000,000) of a meter, which is
the current SIbase unit of length.
It can be written in scientific notation as
1×10−12 m (scientific notation) or 1 E−12 m (engineering notation)
— both meaning 1 m /
1,000,000,000,000.

It equals a millionth of a micrometre (formerly called a micron), and
was formerly called micromicron, stigma, or bicron.[2] The
symbol µµ was once used for it.[3] It is
also a hundredth of an Ångström, an internationally recognized non-SI
unit of length.

The picometre's length is of an order such that its application
is almost entirely confined to particle physics and quantum physics. Atoms are between 62 and 520 pm in diameter.
Smaller units still may be used to describe smaller particles (some
of which are the components of atoms themselves), such as hadrons and fermion point particles.